The stars of the show at PBS Kids include some of the heaviest hitters of the 2-to-8 crowd: the Cat in the Hat, Clifford the Big Red Dog, Curious George and all their public-TV compatriots. They appear in streaming-video versions of their broadcast programs and engaging games. The well-designed user interface doesn’t assume that the individual plopped in front of the computer knows how to read. And although the main site requires Flash Player, there’s also a Flash-free version that works well on the iPad.